The role of environmental context on salmon-benthos interactions in Great Lakes tributaries

Environmental context has recently emerged as an important determinant of Pacific salmon (Oncorhynchus spp.) effects on stream ecosystems.  Factors such as sediment size, stream discharge, and salmon run size determine the magnitude of enrichment or disturbance imparted by salmon on stream biota.  Since environmental factors vary dramatically among geographic regions, salmon effects may be expected to differ in regions were salmon have been introduced, such as the Great Lakes.  We evaluated salmon spawner effects on sediment biofilm in six Great Lakes tributaries and specifically considered environmental context as an explanation for contrasting effects.  We measured streamwater dissolved nutrient concentrations and biofilm chlorophyll a and ash-free dry mass before, during, and after the salmon run.  Dissolved nutrient concentrations increased with salmon abundance during the salmon run.  However, in three streams receiving substantial salmon runs, two exhibited decreased biofilm levels during the run while one showed a marked increase in biofilm.  Differing baseline nitrogen and phosphorus availability among streams suggests that nutrient limitation may explain the contrasting responses of biofilm to salmon.  Nutrient levels were substantially lower where biofilm increased, suggesting salmon may stimulate growth by relieving nutrient limitation.  Our results provide further evidence that ecosystem responses to salmon are highly context-dependent.